Wooden posts and wooden cribs, or chocks, are probably the oldest support systems used in the mining industry. A wooden post, typically 4 inches to 10 inches in diameter or square cross-section, loaded axially provides support between two points. A wooden crib or chock provides support over a larger area, typically varying from a 30 to 72 inches square. Wooden posts and wooden cribs are extensively used in the mining industry even today.
A wood crib consists of layers of two or more parallel timbers with adjoining layers placed at right angles to each other, as shown in FIG. 1. Thus, the number of parallel timbers in each direction determines the number of contact areas through which load is transferred or resisted. For example, a 2 by 2 crib means two layers of timber in each direction, resulting in 4 contact areas. A 2-by-2 crib configuration is most common, although 3-by-3, 3-by-2, and 4-by-4 configurations have been considered and have found limited application.
Underground mines use large number of wooden cribs to provide support over an area between two opposing surfaces. These opposing surfaces are referred to in industry conventions alternatively as to the lower surfaces in mines as floors, footwalls, and as to the upper surfaces as roofs and hanging walls. Typically, cribs are more extensively used in longwall coal mining than in room-and-pillar coal mining. Cribs are also extensively used in non-coal underground mining.
A crib is typically constructed of wooden elements of square or prismatic cross-section, 5 to 6 inches across, although other shapes have also been used. The length of elements used typically varies from 30 inches to 60 inches, depending upon the height of the area to be supported. Aspect ratio for a crib is defined as the ratio of the height of the crib to the distance between centers of contact areas along a timber. Reducing aspect ratio increases the stability of the crib structure, and ratios larger than 2.5 and less than 4.3 are recommended. A crib structure should be designed to have appropriate rigidity, or stiffness, and load carrying capacity to provide early, controlled resistance to rock mass movement to maintain its integrity.
A typical crib uses solid, prismatic wooden crib elements of 5″-by-5″-by-30″ or 6″-by-6″-by-36″, although other sizes may be used. The load is transferred between upper and lower surface areas through typically four contact areas in a horizontal plane of the size 5″-by-5″ or 6″-by-6″ depending on the size of the crib element. Except at and around the contact areas, there is very little stress within the prismatic element. The areas adjacent to the contact areas are in tension while zones away from contact areas have almost no stresses vertically or horizontally. At and below the contact areas are high compressive stresses.
Wood is a transversely isotropic material with much higher strength and stiffness when loaded axially, or parallel to the grain, as compared to loading transversely, or perpendicular to the grain. More specifically, a typical oak timber loaded axially has a compressive strength of 2000-2500 psi, and an elastic modulus of 150,000-250,000 psi. Similar data for the two lateral loading directions are about equal to each other, and are 500-700 psi in compressive strength and 25,000-35,000 psi elastic modulus. Furthermore, the Poisson's ratios for loading in the axial and lateral directions are also significantly different: 0.10-0.20 for loading axially and 0.30-0.40 for loading in the two lateral directions. The wood and numbers here are provided as an example and these may vary.
A typical solid wood cribbing for support has several shortcomings. First, its rigidity is low since wood is loaded at right angles to grain. So, the support column allows a significant amount of deformation, as much as 20% of the total height of the column may be reduced through deformation. Second, because of deformations, the column has limited load carrying capacity, since typical columns are subject to failures from buckling before achieving their full load carrying capacity. Third, air flow in mines is important, and since each cribbing column eliminates about half the available air flow space when installed, because the air is displaced by crib elements, resistance to air flow is significant. Fourth, installing typical solid wood cribbing is difficult in locations where the surfaces are not parallel to each other or irregular. Fifth, each wooden crib element typically weighs about 35 pounds, making carrying them by hand and assembling a cribbing column an arduous process, especially when one must lift above one's head to reach the upper layers. Sixth, since low-rigidity wedges, cut parallel to the wood grain, are typically used to preload the crib, the amount of preload force that can be introduced to a column is limited. The wedges typically deform under light loads, which means the column does not support significant loads until the upper and lower surfaces have collapsed toward each other, compressing the column. Preloading is currently applied through wooden wedges, typically 3 to 4 inches wide that are cut at high incline angles of 10 to 20 degrees. These wedges are loaded transversally to the wood grain and yield at the low pressure of 500 to 700 psi. Since wedges are cut at high incline angles, their contact areas with prismatic crib elements are small. Therefore, stress concentrations at contact points are high and the wedges yield even at low crib loads. The wedges then become loose providing little or no preload on the installed crib. Industry professionals suggest that there is a need to develop a relatively simple mechanism to apply a sustained preload of 5 to 8 tons when a crib is installed.
U.S. Pat. No. 6,352,392 describes a mine roof support crib. The crib includes a plurality of chocks that are connected together through notches in the chocks to form only three planes with at least two of the planes in perpendicular relation with each other and able to support at least five tons of load. Alternatively, the plurality of chocks that are connected together through notches in the chocks form only two planes which are in perpendicular relation with each other and are able to support at least five tons of load. The invention uses solid wood elements, and when assembled forms surfaces through which air flow are not possible. Furthermore, the elements are loaded transversally, requiring more material to support a given load.
U.S. Pat. No. 5,746,547 is concerned with a mine support crib of the type which comprises a series of superimposed layers of elongate chocks. There is a plurality of parallel, spaced apart chocks in each layer with the chocks in one layer arranged transversely to the chocks in the adjacent layer or layers so that the chocks in a given layer, other than the bottom layer, cross the chocks in the layer below at crossing points which are located inwardly of the ends of the chocks. According to the invention, operatively upper and lower surfaces of the chocks are formed with notches at the crossing points. The notches interlock with one another to lock the chocks together. The notches are of such depth that portions of the chocks which are located between and beyond the notches bear on corresponding portions of chocks in the next layer but one below. The invention has the disadvantage of creating vertical surfaces which are impervious to air flow, and which utilize significant amounts of timber to support a given load, in part because the crib elements are loaded transversally to the grain direction.
U.S. Pat. No. 5,435,670 describes a method and apparatus for providing a crib type yielding support between two areas in mines. The support consists basically of three elements: a spacer, a pack, and a grout-inflatable bag. The spacer is designed to be stiffer than the pack. The pack is designed to have lower stiffness and yield under loading. As the name implies, the spacer basically fills in the height to maintain the slenderness ratio within limits. A spacer may consist of interconnected elongate timbers side-by-side such that loading on it is parallel to the wood grain. Another spacer configuration taught by the patent includes forming a layer of timbers with some elongate timbers with wood grain parallel to loading and others perpendicular to loading. Multiple layers are stacked on the top of each other with multiple contact points through which load is transferred. The pack also consists of network of elongate timbers laid out in a manner that the pack has low stiffness and is much more compressible than the spacer and has yielding characteristics. A grout-inflatable bag is used to provide a preload to the entire system by expanding against the roof or the floor in a coal mine or the hanging wall and the footwall in coal or non-coal mine. The inflatable bag conforms to uneven roof and applies preload to the support assembly. The invention also uses significant amounts of raw materials to create surfaces through which air flow is not possible, because the elements must abut and be interconnected to each other to resist buckling. Also, each layer of spacer is preformed and transported to installation site. For such a crib assembly, material handling is difficult because of the weight and size of the pre-assembled structure.
U.S. Pat. No. 4,628,658 discloses an interconnected cribbing system. The described cribbing system for supporting a mine roof or the like includes a multi-sided column. Each column is formed of a stack of cribbing elements with each cribbing element having upper and lower surfaces wherein substantially the entire upper and lower surface of each cribbing element is a bearing surface for transmitting a substantially vertical load to a vertically adjacent cribbing element of one stack. The majority of loading forces are transmitted in this invention transverse to grain direction, and the structure creates a vertical surface that obstructs airflow, however.